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Myasthenia Gravis - A Comprehensive Overview

Definition

Myasthenia gravis (MG) is the most common primary disorder of neuromuscular transmission (NMT). Autoantibodies bind to proteins at the postsynaptic neuromuscular junction (NMJ) - most commonly the nicotinic acetylcholine receptor (AChR) - disrupting the conversion of nerve impulses into muscle fiber action potentials. The result is fatigable muscle weakness that worsens with activity and improves with rest.

Bradley and Daroff's Neurology in Clinical Practice, p. 2770

Epidemiology

US prevalence: ~20/100,000 (approx. 60,000 patients)
Prevalence has increased over the past 50 years (better ascertainment, reduced mortality, aging population)
Sex and age pattern: Women are affected ~3x more often than men before age 40; incidence is higher in males after age 50; roughly equal during puberty
Today, the majority of US MG patients are over age 50 and more men are now affected than women overall
Bradley and Daroff's Neurology in Clinical Practice, p. 2770

Pathophysiology

The NMJ has no blood-nerve barrier, making it uniquely vulnerable to circulating immune factors. Three antibody mechanisms have been described:

Antibody Target	Mechanism	Prevalence
AChR (alpha-1 subunit)	Complement activation destroying the postjunctional membrane; antigenic modulation (cross-linking + receptor internalization); direct functional blockade	~85% of generalized MG
MuSK (muscle-specific kinase)	IgG4 antibodies inhibit clustering of AChRs on the postsynaptic surface	Up to 50% of AChR-negative generalized MG
LRP4 (lipoprotein receptor-related protein 4)	Interferes with signals maintaining NMJ integrity	Minority; less well understood
Seronegative	No detectable antibodies by standard testing	~10%

The net result is a reduced density of functional AChRs at the motor endplate. With repeated nerve stimulation, the normal presynaptic quantal release of ACh decrements against a shrinking pool of available receptors, producing the characteristic fatigable weakness.

The thymus is central to pathogenesis:

Early-onset MG (mostly women): thymic hyperplasia with active germinal centers suggests a thymus-initiated breakdown in immune tolerance
Thymoma-associated MG (~10-15% of patients): deficiency of the autoimmune regulatory protein AIRE leads to reduced regulatory T cells and increased autoreactivity
There is also a genetic component - ~5% of patients have an affected family member, with a heritability index of 0.65
Goldman-Cecil Medicine, p. 4111; Bradley and Daroff's Neurology in Clinical Practice

Clinical Presentation

Cardinal Features

Ocular motility abnormalities in Myasthenia Gravis - ptosis and ophthalmoplegia from fatigable weakness of periocular muscles

Fig: Ocular motility abnormalities in MG. A-B: Progressive right lid ptosis on sustained forward gaze. C: Incomplete superior movement on attempted upward gaze. D-E: Skew deviation and incomplete lateral gaze from multiple periocular muscle weakness. (Bradley and Daroff's Neurology in Clinical Practice)

Ptosis and/or diplopia are the initial symptoms in up to 85% of patients; nearly all develop both within 2 years
Weakness worsens with activity and improves with rest; typically least severe in the morning
Bulbar symptoms (dysphagia, dysarthria, jaw fatigue) affect up to 30% at onset
Limb weakness (proximal > distal) occurs in ~10% at onset
Respiratory muscles can be involved - leading to myasthenic crisis

Distribution Patterns by Subtype

Subtype	Key Features
Ocular MG (OMG)	Confined to ocular muscles; 10-15% in Caucasians, up to 58% in Asian populations; if ocular only after 2 years, 90% chance it won't generalize
Early-onset generalized (EOMG)	Women, <50 y, AChR-Ab+, thymic hyperplasia
Late-onset generalized (LOMG)	Men, >50 y, AChR-Ab+, normal/atrophic thymus; anti-titin and anti-ryanodine receptor Abs common; more crises
MuSK-MG	Predominantly female; prominent bulbar/facial/neck/respiratory weakness; often muscle atrophy; may worsen with pyridostigmine
Thymoma-associated	Equal sex; peak onset ~50 y; striated muscle Abs common

Bradley and Daroff's Neurology in Clinical Practice, p. 2771-2774

Diagnosis

1. Clinical recognition: Fatigable weakness in a characteristic distribution - ptosis worsening on sustained upgaze, fatigable proximal limb weakness, bulbar findings.

2. Serological testing:

AChR antibodies (binding, blocking, modulating) - positive in ~85% of generalized MG, ~50% of ocular MG
MuSK antibodies - checked when AChR-Ab negative
LRP4 antibodies - emerging test

3. Edrophonium (Tensilon) test: IV injection of this short-acting acetylcholinesterase inhibitor produces transient improvement in ocular or bulbar weakness; now less used due to cardiac risks.

4. Electrodiagnostic studies:

Repetitive nerve stimulation (RNS): Decremental response (>10% decrement) at 3 Hz is characteristic
Single-fiber EMG (SFEMG): Most sensitive test; increased "jitter" reflects variable NMJ transmission; required when RNS and antibodies are negative (especially in OMG)

5. Imaging:

CT or MRI of chest to detect thymoma or thymic hyperplasia - required in all newly diagnosed MG patients
Wills Eye Manual; Bradley and Daroff's Neurology in Clinical Practice; Goldman-Cecil Medicine

Myasthenic Crisis

A myasthenic crisis is respiratory failure from respiratory or oropharyngeal muscle weakness requiring ventilatory support. It is a medical emergency. Common triggers include:

Infection (especially respiratory)
Aspiration/dysphagia (precipitant in 56% of crises in one study)
Medication errors or non-compliance
Aminoglycosides, fluoroquinolones, beta-blockers, magnesium, neuromuscular blocking agents (all can worsen MG)
Surgery, pregnancy, stress

Management requires ICU admission, ventilatory support, plasmapheresis or IVIg (begin before high-dose steroids to prevent early steroid-associated worsening), and treatment of the underlying trigger.

Treatment

Treatment is individualized by extent (ocular vs. generalized) and severity. The goal defined by the MG Foundation of America Task Force: "no symptoms or functional limitations from MG, with no more than mild side effects requiring no intervention."

1. Symptomatic: Cholinesterase Inhibitors (ChEIs)

Pyridostigmine (Mestinon): 30-60 mg orally every 4-8 hours; first-line symptomatic therapy for most patients
Inhibits acetylcholinesterase, increasing ACh availability at the NMJ
Muscarinic side effects: nausea, diarrhea, cramps, increased secretions - can be managed with glycopyrrolate
Caution: MuSK-MG patients may worsen with ChEIs

2. Short-term Immunotherapy (rapid but transient benefit)

Plasma exchange (plasmapheresis): Removes circulating antibodies; response within days; used pre-operatively, pre-thymectomy, and in crisis
IVIg: Comparable efficacy to plasma exchange; mechanism includes Fc receptor saturation and immune modulation; given as 2 g/kg over 2-5 days

3. Long-term Immunosuppression

Agent	Mechanism	Notes
Prednisone	Broad immunosuppression	First-line; start low (15-25 mg/d) and increase to 50-60 mg/d; taper slowly once controlled; watch for early worsening
Azathioprine	Purine synthesis inhibition	RCT evidence; steroid-sparing; onset 12+ months; check TPMT enzyme; avoid with allopurinol
Mycophenolate mofetil (MMF)	Selective lymphocyte purine synthesis inhibition	1000-1500 mg twice daily; widely used; teratogenic - stop 4 months before pregnancy
Cyclosporine	Calcineurin inhibitor (inhibits IL-2)	RCT evidence; limited by nephrotoxicity and drug interactions
Tacrolimus	Calcineurin inhibitor	Approved for MG in Japan; useful in refractory cases; less nephrotoxic than cyclosporine
Rituximab	Anti-CD20 B-cell depletion	Particularly effective in MuSK-MG; a 2025 Cochrane review (PMID: 40607605) assessed its evidence base
Cyclophosphamide	Alkylating cytotoxic agent	Reserved for refractory cases; RCT evidence for pulse IV dosing

4. Thymectomy

Recommended for all patients with thymoma
Also recommended for AChR-Ab+ generalized MG patients aged 18-65, even without thymoma (MGTX trial evidence)
Effect is gradual - maximal benefit may not appear for years

5. Novel Biologics (emerging/approved)

Recent network meta-analyses (PMID: 40346603, PMID: 39470879) have evaluated newer agents:

Eculizumab (complement C5 inhibitor): FDA-approved for refractory generalized AChR-Ab+ MG
Efgartigimod (FcRn antagonist): Reduces IgG antibody levels by blocking neonatal Fc receptor recycling; FDA-approved
Rozanolixizumab, nipocalimab: Also FcRn antagonists under study
Zilucoplan: Subcutaneous complement C5 inhibitor; FDA-approved 2023

These agents represent a mechanistic shift from broad immunosuppression to targeted pathway blockade.

Bradley and Daroff's Neurology in Clinical Practice, pp. 2775-2790; Harrison's Principles of Internal Medicine 22E, p. 3674

Prognosis

The prognosis has dramatically improved in the past 50 years due to advances in ICU care and immunotherapy. With modern treatment:

The majority of patients achieve minimal manifestation status or complete stable remission
Mortality from MG itself is now rare in specialized centers
Spontaneous remission occurs in a subset, more common in early-onset ocular MG
Patients with thymoma have a more guarded prognosis related to the underlying neoplasm

Summary Table: MG vs. Lambert-Eaton (key differential)

Feature	Myasthenia Gravis	Lambert-Eaton Myasthenic Syndrome
Antibody target	AChR / MuSK / LRP4	VGCC (voltage-gated calcium channel)
Weakness pattern	Ocular, bulbar, proximal limb	Proximal limb (legs >> arms); spares ocular
Fatigability	Worsens with use	Improves transiently with brief exercise
Reflexes	Normal	Reduced, may augment after exercise
RNS	Decremental at low frequency (3Hz)	Incremental at high frequency (>10Hz)
Autonomic features	Absent	Common (dry mouth, constipation)
Association	Thymic disease	Small cell lung cancer (60%)

Recent evidence note: A 2025 Cochrane systematic review (PMID: 40607605) examined rituximab specifically in MG, and a 2025 network meta-analysis (PMID: 40346603) compared novel biologics (complement inhibitors, FcRn antagonists) - both informing the rapidly evolving treatment landscape for generalized MG.

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